Chip resistors are typically screen printed as thick film pastes on a large, square alumina substrate with as many as a thousand chip resistors on a single such substrate. The printed resistors are then fired to remove all of the organic medium from the printed pattern and to densify the solids. A first encapsulant glass layer is printed over the resistors and fired. The resistor values at this point have a distribution of 3-5%. The once encapsulated resistors are trimmed with a laser beam directly through the encapsulant, and the printed resistor layer, and into the alumina substrate. The laser trimming increases resistance values about 50%, but reduces the distribution of resistance values to about 0.1%
After laser trimming through the first encapsulant layer and the resistor, a second glass encapsulant is printed over the trimmed resistor and fired at 600.degree. C. After firing the second encapsulant layer, the large substrate is broken into strips and a conductive edge termination is applied by dipping the edge of the strips into a conductive paste. The thusly terminated strips are then fired. After firing the edge terminations, the strips are broken into individual chips and the chip terminations are nickel and solder-plated. The finished chip resistors are about the size of a large grain of sand. They are usually soldered to a printed wiring board for use.
Chip resistors such as those described above are frequently made in a wide range of resistances from 1 to 1,000,000 ohms, and to be effective, they must have a resistance shift upon encapsulation and trimming of no more than 0.5%. Resistance stabilities such as this, however, are very difficult to achieve with low-end resistors, i.e., those having resistance values of only 1-100 ohms per square.
Low-end resistance resistors of the current state-of-the-art, such as those based on RuO.sub.2 alone, tend to have resistance shifts exceeding 0.5% in 1000 hours after laser trimming, whereas higher resistance resistors are much more stable. In addition, the state-of-the-art low resistance resistors are traditionally difficult to manufacture to a resistance of .+-.10% and a temperature coefficient of resistance (TCR) of .+-.100 ppm/.degree. C. because a dense, consistent, insensitive microstructure is difficult to achieve. The relatively low volume fraction of glass binder phase in such compositions makes it difficult to achieve this desired dense, consistent microstructure.